RFR: Use @mentions to improve email threading

[Robin Westberg]

Hi all,

Please review this change that looks for an `@mention` marker in comments, to select which comment to use as the parent
when creating a threaded email.
Best regards,
Robin

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Commit messages:
 - Add support for review comment threads as well
 - Find mentions in plain comments

Changes: https://git.openjdk.java.net/skara/pull/486/files
 Webrev: https://webrevs.openjdk.java.net/skara/486/webrev.00
  Stats: 161 lines in 2 files changed: 160 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/486.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/486/head:pull/486

PR: https://git.openjdk.java.net/skara/pull/486